Earlier quoted context omitted.
Why would it be humid? Heat doesn't cause humidity. Deserts can be hot and dry. (Edited are to can be for the pedantic people.)
I think standard household activity would generate a lot of humidity. Usually dwellings vent outside, but here the outside is in a larger "inside". I'm guessing the greenhouse has its own air regulation set up.
We have a MHRV system, but it doesn't account for humidity, so will happily replace all the air inside with dry air from outside (I'm trying to automate it with HASS). Our bathroom fan has a humidity sensor, so doesn't run so much in the winter, but even so the humidity often gets down to 25% rh inside.